1. Field of the Invention
The present invention relates to an optical transmission module and particularly to an optical transmission module in which a connection method of a high frequency transmission cable is utilized.
2. Related Background Art
An optical transmission module includes components mounted on different circuit boards. These components are connected with a bonding wire or a coaxial cable.
When these components are connected with a bonding wire, an inductance increases according to the length of the bonding wire (connection distance). Therefore, this bonding wire is not suitable for transmission of a high frequency signal.
Meanwhile, since a coaxial cable is covered with an external conductor, when the connection distance becomes longer, an inductance does not become larger than that of a bonding wire. Therefore, connection by a coaxial cable is suitable for transmission of a high frequency signal.
In the case where a coaxial cable is connected to a coplanar transmission line, a core wire which appears when the external conductor of a coaxial cable is peeled is connected to a signal line of the coplanar transmission line. Therefore, the external conductor does not exist in a certain area of the circumference of the core wire of a coaxial cable. In the circumferential area covered with the external conductor, a uniform electromagnetic field is generated in the direction of normal line from the signal wire. However, in the boundary to the area where the external conductor does not exist in the circumferential area of the core wire of a coaxial cable, direction of the electromagnetic field changes extremely. Extreme change of the direction of electromagnetic field causes degradation of transmission efficiency of an high frequency signal. Moreover, since the thin external conductor of a coaxial cable is spreading in the circumferential area of the signal wire, it has been difficult to realize the connection of such external conductor with a ground electrode of the coplanar transmission line substrate.
As a method of improving the transmission efficiency of a high frequency signal and also improving the connection property, there is proposed a method of using a connector.
Referring to FIGS. 10A and 10B, a connection structure between a coaxial cable and a coplanar transmission line using a connector will be explained.
As illustrated in FIG. 10A, a male connector S1 connected to an external conductor of coaxial cable is mounted at one end of the coaxial cable and a female connector S2 is formed at a part of the side wall of a housing to which a coplanar transmission line is mounted. The male connector S1 has the diameter of about 3.8 mm (FIG. 10B) to 9 mm (FIG. 10A) and is soldered with the external conductor of the coaxial cable. A core wire of the coaxial cable is led out from one end thereof. Since the female connector S2 is required to have a constant thickness in order to fix the male connector S1, this connector S2 is formed thicker than the male connector S1. Therefore, this female connector S2 is fixed by soldering process to a ground electrode of a coplanar transmission line by utilizing such thickness.
An optical transmission module in future tends to have a thin structure but thickness of connector has been considered as a cause which disables realization of thin structure of an optical transmission module as a whole. Therefore, the inventors of the present invention have discussed a small thickness structure of a connector.
The inventors of the present invention has proposed a structure of a connector in which a connector is formed of one member in place of forming a structure composed of a male connector and a female connector. Namely, a connector (contact sleeve) is formed of an individual thin cylindrical member which is not a part of a housing.
As a result of actual connection using such contact sleeve, it has been proved that when the contact sleeve is formed thinner, poor connection is easily generated in the solder connection to the ground electrode of a coplanar transmission line. It has also become obvious that if such poor connection is generated, a signal wire as the core wire of a coaxial cable is no longer covered with an external conductor at a certain area and a fine potential difference is generated in the ground potential of a ground wire between the contact sleeve and the coplanar transmission line. Thereby, an electric field centralized toward the respective ground electrodes of the contact sleeve and coplanar transmission line in such area of the coaxial cable not covered with the external conductor and the electromagnetic field is deflected due to such potential difference. Accordingly, the transmission efficiency of a high frequency signal has been lowered.